Recently, there has existed increasing interest in energy storage technology. Batteries have been widely used as energy sources in portable phones, camcorders, notebook computers, PCs and electric cars, resulting in intensive research and development for them. In this regard, electrochemical devices are the subject of great interest. Particularly, development of rechargeable secondary batteries is the focus of attention. Recently, continuous studies have been performed to develop a novel electrode and battery having an improved level of capacity density and specific energy.
Among the currently used secondary batteries, lithium secondary batteries, developed in early 1990's, have a drive voltage and an energy density higher than those of conventional batteries using aqueous electrolytes (such as Ni—MH batteries, Ni—Cd batteries and H2SO4—Pb batteries), and thus are spotlighted in the field of secondary batteries. However, lithium secondary batteries have a problem related to their safety, due to ignition and explosion caused by the use of an organic electrolyte. Also, lithium secondary batteries have a disadvantage in that they are manufactured via a relatively complicated manufacturing process. A lithium ion polymer battery, developed recently, can improve the aforementioned problem and disadvantage of a lithium ion battery, and thus has been regarded as one of the most potent candidates for next-generation batteries. However, a lithium ion polymer battery shows a relatively lower capacity as compared to a lithium ion battery, and has an insufficient discharge capacity, particularly at low temperature. Meanwhile, since the capacity of a battery is in proportion to the content of an electrode active material, it is very important to design a cell structure in such a manner that as much an electrode active material as possible can be packed in a limited space provided by a battery pack.
To solve the aforementioned problems, Korean Laid-Open Patent Nos. 2001-0082058 and 2001-0082059 disclose an electrochemical device, which is manufactured with ease and has an efficient space structure sufficient to maximize the amount of an electrode active material, the electrochemical device comprising multiply stacked electrochemical cells. Herein, the basic unit of the stacked electrochemical cells is a full cell or a bicell.
Characteristics of a full cell or a bicell largely depend on the electrode, electrolyte and other materials used therein. Particularly, because the content of the active material introduced into an electrode is ultimately related to the maximum amount of lithium ions capable of binding, the capacity of a battery is in proportion to the content of an electrode active material. Hence, if a binder having excellent adhesion so as to reduce the amount of a binder, it is possible to provide an electrode comprising an active material in an amount increased accordingly. Therefore, there has been a need for a novel binder having excellent adhesion.
Meanwhile, commercially available binders for an electrode include polyvinylidene fluoride (PVDF)-based polymers. More particularly, PVDF-based polymers include PVDF homopolymer, and PVDF copolymers such as polyvinylidene fluoride-hexafluoropropylene copolymer (Korean Laid-Open Patent No. 2001-0055968) and polyvinylidene fluoride-chlorotrifluoroethylene copolymer. Such PVDF-based polymers are advantageous in that they are chemically and electrochemically stable. However, PVDF-based polymers are problematic in that they should be dissolved in an organic solvent such as NMP (N-methyl-2-pyrrolidone) and then used in the form of a binder composition, thereby causing an environmental problem; they show poor safety; and have low affinity to a liquid electrolyte, thereby causing degradation in the quality of an electrode. Additionally, although PVDF-based polymers perform their functions by surrounding an electrode active material and thus show a good binding property to inorganic particles, including electrode active material particles, they show low adhesion to a metal such as a collector. Hence, PVDF-based polymers have an additional disadvantage in that they are used in a great amount in order to exert and maintain sufficient adhesion to a metal, etc.
To solve the aforementioned problems, there has been an attempt to provide an aqueous binder, which uses water as a dispersion medium and is used in a smaller amount (JP 2872354, JP 3101775 and KR 2000-0075953). However, such aqueous binders show insufficient adhesion between a collector and an electrode active material, thereby providing poor quality. Moreover, when a thermal bonding process is performed between an electrode and a separator in order to provide a high-capacity electrochemical device comprising multiply stacked electrochemical cells, such aqueous binders cannot show sufficient adhesion, resulting in degradation in the overall quality of a battery. Therefore, there is a need for a novel binder, which shows sufficient adhesion between an electrode active material and a collector, and permits thermal bonding between an electrode and a separator so as to provide multiply stacked electrochemical cells.